THE ONE WHERE Arthur marches to Camlann to face Morgana's army head on; but Merlin can't join him because he's lost his powers. Can the Crystal Cave restore them in time?

VERDICT So, Merlin's most powerful form is old man Emrys? It's hardly like changing into a Superman outfit. An unexpected end to a dark, thoughtful episode. In terms of giving him the traditional appearance of Merlin at the final battle I can understand why, and perhaps Merlin choose this disguise because as far as he knows Arthur's still unaware of his magic. But it doesn't feel like Merlin's authentic form and it's hardly a punch-the-air badass moment to end on.

In keeping with the recent run, this is a solid Merlin tale with some superb touches that just stops short of being amazing; despite some sharp ideas and splendid performances (Colin Morgan sobbing while trapped in the cave, for instance) it doesn't quite go far enough, although we gain an insight into how much the king values him. And Arthur gives a rousing speech which isn't quite Henry V but which is nevertheless better than many such morale-boosters - for instance the undercooked one at the end of Snow White And The Huntsman earlier this year.

Merlin without magic is an interesting concept, but it serves little purpose except to separate him from Camelot for a time and create an excuse for a pep talk. And it begs the question, why does Morgana go to this trouble to take Merlin's magic? Surely it would be easier just to kill him? Or expose him so that Arthur has to reject him? Once again Morgana's plan is too complicated. When she catches up with him she just traps him in the Crystal Cave instead of finishing him off. She's like the worst kind of Bond villain.

But the episode does hang together. Isn't it creepy to see Mordred at the right hand of Morgana's throne? He doesn't look overly happy with his new mistress's methods; his face betrays him (I'm going to miss Alexander Vlahos when this series is over) and even challenges her: "He was a loyal soldier - in robbing him of his magic you've lost yourself a powerful ally." It's terrific to see Morgana using some properly ground-shaking magic like that fireball spell. In comparison we see Merlin, um, create a butterfly in his hands like a parlour trick, but at least there's more than just the classic air punch this time.

The tender heart of this episode is, as always, Merlin's relationship with Arthur. About 19 minutes in there's a touching farewell scene between the two of them as Merlin reveals he has to go on an errand instead of join the battle. The emotion on their faces reveals a degree of heartbreak as Arthur realises Merlin won't be there, and Merlin can't reveal how much he's actually trying to help. The king confesses that he never meant all those jokes he made at Merlin's expense: "I always thought you were the bravest man I ever met" he tells his servant. This is a sad day for the bromance and a bit harsh from Arthur - if he truly has admired Merlin in the past and if he genuinely does trust him then he should have faith that Merlin's "chore" is important.

The final moments of the episode, with the two armies clashing by torchlight, demonstrates that Merlin is still capable of giving us epic fantasy battles. And we only have to wait 48 hours for the conclusion! Start the countdown...

INFLUENCES This episode is like an I Spy book of cinematic influences. It pays homage to some of the best fantasy tropes on the screen. Fortunately it works, but they are blatant. First off, Morgana's slug thing owes a lot to the Alien facehugger. Then Merlin's father appearing to him as a glowing blue figure smacks of Obi-Wan Kenobi. Our hero losing his powers and having to get it back from crystals is the stuff Superman II is made of (Kal-El speaks to his dead father for support at his Fortress Of Solitude too). Meanwhile the gathering of troops at the narrow pass is pure 300 , a comparison strengthened by the stylised quick-slow-quick combat - Morgana's sneaky back route is even a turning point in the battle of Thermopylae. When Arthur stood up to make his rallying speech in the torchlight I was convinced he was about to say, " Spartans! Tonight we dine in hell !"

NITPICK How will Gwaine look after himself on the way back from the valley, and indeed how will he fight in the battle, if Merlin has his sword?

WHOSE SIDE ARE YOU ON ANYWAY? I can't help thinking that Morgana's anti-magic slug is exactly the sort of thing that Uther would have loved - it destroys sorcerers!

IT'S WOSSERNAME AND WOSSISNAME AND ALSO THINGY The luckless Ari, on whom Morgana tests her magic-scoffing face-slug, is played by Peter Guinness; he's been in loads of stuff but you might recall him from Alien 3 . The survivor of the village raid, Eira, who hooks up with Gwaine back in Camelot is played by Erin Richards. ( Merlin has a bit of a tradition of portraying women as deceivers so it's no surprise that Eira turns out to be a wrong 'un.) Welsh actress Richards is perhaps best known for playing the detective investigating the Box Tunnel massacre in Being Human . Meanwhile it's a welcome return for John Lynch as Merlin's long-lost father. We last saw him in the season two episode "The Last Dragonlord". He's been in a number of films and TV shows, but you'll surely remember him as Gwyneth Paltrow's boyfriend in Sliding Doors .

LOST IN TRANSLATION Gaius says that the box bearing the ugly slug creature is decorated with the mark of the Gean Canach, "in the time of the old religion... the most feared creation of the magic folk". Actually Gean Cánach means "love talker" in Irish and a Gancanagh is typically a seductive male fairy, a sort of charming incubus.

CLARK KENT WOULDN'T DO THAT Merlin uses magic to cheat at gambling, making Arthur look bad in front of the knights during the dice game. It's funny but it's also the behaviour of a git.

SLASH BAIT Precious little, beyond the understated thoughtful moments between Arthur and Merlin, so the 'shipping forecast is short this week. The only shirtless knight we see in this episode is Gwaine, suggestively sitting on a bed with Eira.

THE LEGEND Tonight Morgana gives Mordred a sword forged in the breath of a dragon. Sure enough, Excalibur is by no means the only magic sword in the traditional stories. In fact the Middle English Alliterative Morte Arthure talks of Mordred going into the final battle wielding a sword called Clarent, a weapon that had once been used for official ceremonies at Camelot.

BEST LINE Gaius: "If Arthur goes to Camlann the prophesy will come true and he will die."